Disease name: Rhizosphaera needle cast
Pathogen name: Rhizosphaera kalkhoffii (fungus)
Hosts: Rhizosphaera needle cast is a common needle disease of spruces and other conifers. Colorado blue spruce is particularly susceptible and can be severely damaged by this disease. Other hosts include white, black, Engelmann, and Serbian spruce; Austrian, mugo, Eastern white, and Japanese red pine; as well as Douglas-fir. Norway spruce is relatively resistant.
Symptoms
Infection generally begins in spring on the needles of the lower branches, soon after the needles have elongated. Symptoms spread upward and around the tree. Sometimes infection will start on branches in the middle of a tree, creating defoliated areas among healthy branches.
Although infection occurs in spring, it will be many months before symptoms are visible. During late summer, this disease causes first-year needles to appear mottled or speckled with dull yellow or reddish blotches.
Later, often during the next year, infected needles on the interior of a branch turn purplish-brown (from the tips downward) and drop prematurely.
Heavily infected trees can suffer severe needle loss and branches may die as they become defoliated.
Trees are rarely killed by Rhizosphaera needle cast, but several years of attack will take its toll, and only the current season needles may remain. This reduces the value of the infected tree as a landscape specimen.
Disease Cycle
The fungal pathogen that causes Rhizosphaera overwinters as both vegetative mycelium and spores on infected and dead needles. In spring, under moist conditions, the spores are dispersed and will infect the newly emerging needles.
Although the infection occurs in spring at the time of new growth, symptoms will take several months to develop and needles may not drop until 12 to 15 months after infection.
In the autumn after infection, or in the following spring, fruiting bodies will develop in the infected needles. They appear as distinct rows of black, pinhead-size dots. These fruiting structures emerge through needle pores (stomata). This can be seen with a hand lens or by the unaided eye. Healthy stomata appear white. Those in which the fruiting bodies develop will be black. The rows of black stomata are a diagnostic feature of Rhizosphaera needle cast.
Management
Cultural Management
Working near trees in wet weather should be avoided throughout the growing season. For all trees showing symptoms, remove, when feasible, dead branches, fallen needles, and cones under the tree to prevent further infections. Prune surrounding plants to promote better air circulation and keep plants well watered, especially in periods of drought, to alleviate stress. Water all evergreens before winter to avoid root desiccation and winter injury.
Chemical Management
Fungicides are available to treat this disease.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For current pesticide recommendations, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.